1. Field of the Invention
The present invention relates to a recording medium which uses a light as a recording source and performs visually readable recording by utilizing a material capable of emitting heat after thermal conversion of the absorbed light.
2. Description of the Prior Art
In a direct recording system which does not require development and fixing steps, heat-sensitive recording papers which use basic dyes and organic developers as couplers are excellent in operation and maintenance. Thus, the heat-sensitive recording papers are widely used for facsimiles and printers.
In this system, however, hot recording is conducted by bringing a thermal head or an exothermic IC pen into direct contact with a heat-sensitive recording paper. Consequently, colored substances in a fused state adhere to the thermal head or the exothermic IC pen, cause troubles such as dregs adhesion and sticking, and lead to problems of record obstruction and impairment of record quality.
In particular, when a line is continuously drawn in a recording direction as in the case of a plotter printer, it has been impossible to carry out continuous printing without trouble of dregs adhesion.
Further, in the recording system using the thermal head, it has been considered difficult to heighten an image resolution to 8 dotts/mm or more.
Consequently, non-contact recording systems using light have been proposed as techniques for eliminating the troubles of dregs adhesion and sticking and further improving image resolution.
Japanese Patent Laid-Open Publication No. 4142/1979 discloses a heat-sensitive recording medium prepared by coating a substrate with a heat-sensitive recording layer essentially consisting of a leuco dye, which comprises using a metallic compound having a lattice defect. The metallic compound absorbs beams in visible and infrared regions, performs thermal conversion and enables heat-sensitive recording.
Japanese Patent Laid-Open Publication No. 209594/1983 discloses an optical recording medium prepared by lamianting on a substrate at least one set of layer consisting of a near infrared absorbent layer having an absorption wave length in a near infrared region of 0.8 to 2 .mu.m and a heat-sensitive color-developing material layer. Japanese Patent Laid-Open publication No. 94494/1983 describes a recording medium obtained by coating a substrate with one or more of heat-sensitive color-developing materials and one or more of near infrared absorbents composed of compounds having a maximum absorption wave length in near infrared region of 0.7 to 3 .mu.m. It is disclosed that recording on these recording media can be carried out by use of a hot plate or laser beams having a wave length in the vicinity of near infrared region.
Japanese Patent Laid-Open Publication Nos. 94494/1983 and 209594/1983 describe a method for obtaining a light-absorbing heat-sensitive color-developing layer by applying and drying a coating compound prepared by direct addition of a near infrared absorbent to a heat-sensitive color-developing material. These methods, however, exert desensitizing effect or coloring effect on the heat-sensitive color-developing material. Most of these methods impair color development and the ground color, and hence it is difficult and impractical to select the near infrared absorbing material capable of being used. As a countermeasure to the problem, a method has been proposed to alternately laminate a layer containing the near infrared absorbent and another layer containing the heat-sensitive color-developing material. However, increase in number of lamination is disadvantageous in view of production.
The near infrared absorbents disclosed in these publications are cyanine dyes, thiol nickel complexes and squalium dyes. Other near infrared absorbents which have been known are nitroso compounds and metal complexes thereof, polymethine dyestuffs (cyanine dyestuffs), cobalt or palladium complexes of thiol, phthalocyanine dyestuffs, triallyl methane dyestuffs, immonium or diimmonium dyestuffs and naphthoquinone dyestuffs. These near infrared absorbents are described in the paper entitled "Near Infrared Absorption Dyestuffs" [Chemical Industry (Japan), 43, May 1986].
Any of the above conventional near infrared absorbents are remarkably colored and provides poor ground color for the optical recording medium containing the near infrared absorbents as such. In addition, when these near infrared absorbents are directly contained in the heat-sensitive color-developing materials, any of these absorbents exhibit desensitizing action and hence color density is insufficient.
When the optical recording medium is photodiscs, reading is mechanically made and hence an enough optical contrast obtained in a reading wave length serves to carry out reading. However, a readable contrast between image density and ground color is required for the direct reading of recorded information with naked eyes of human. The above methods which have been known for using the near infrared absorbents cannot provide sufficient contrast.